1. Field of the Invention
The present invention relates to a toner for developing latent electrostatic images in the field of electrophotography, electrostatic recording or electrostatic printing.
2. Discussion of Background
In the electrophotographic process and electrostatic recording or printing process, a latent electrostatic image is formed on a latent-electrostatic-image bearing member such as a photoconductor or a dielectric material and developed into a visible toner image with a toner which comprises toner particles pulverized to an appropriate particle size.
For the above-mentioned development, the toner is formed into a thin toner layer by a regulating member such as a blade on a toner-supply-roller such as a development sleeve, and charged to a predetermined polarity in a development unit. The electrostatic images formed on the latent-electrostatic-image bearing member are developed into visible toner images. The toner images thus obtained are then transferred to an image-receiving medium such as a sheet of paper or a plastic film when necessary.
The transferred toner images are then fixed on the image-receiving medium, for instance, with the application of heat and/or pressure thereto, or by the application of a solvent vapor thereto. Toner-image-bearing copies are thus obtained.
In recent years, in accordances with the remarkable development of office automation, the demand for easy and speedy color image copying is increasing, particularly color image copying which is capable of providing colored copy images faithful to the original color images just like printed color images. In order to meet this demand, it is required to produce toner particles which have a small particle diameter and a low melt viscosity and are capable of being fixed on the image-receiving medium at low temperatures.
Conventionally, a toner for use in an electrophotographic developer is prepared by kneading a mixture of a binder resin, a coloring agent, a charge controlling agent, a fluidity-imparting agent and a grinding assistant, pulverizing the kneaded mixture to obtain finely-divided particles, and classifying the finely-divided particles. This method is hereinafter referred to as a pulverizing method. Other methods such as a polymerization method are also employed to obtain toner particles.
Various kinds of toner particles are prepared by the pulverizing method, as disclosed in Japanese Laid-Open Patent Applications 60-263951, 61-240252, 3-94269 and 4-127164. The toners obtained by this method comprise as the binder resin component a resin with a low softening point, such as polymer resin, polyol resin or epoxy resin, which is used alone or in combination with a styrene-based resin in order to achieve the image-fixing at low temperature. When the above-mentioned toners are used, however, the problems of a so-called off-set phenomenon by which fixed toner images are thermally transferred back to members other than the image receiving medium and the curling of fixed toner images tend to occur. These are serious problems in the copy making process.
On the other hand, when the toner particles are prepared by the polymerization method, as disclosed in Japanese Laid-Open Patent Applications 59-218460 and 61-46955, a large quantity of wax can be added to polymerizable monomers during the polymerization thereof for producing binder resin particles, whereby the occurrence of the off-set phenomenon can be efficiently prevented, but the image-fixing at low temperature is hindered.
The toner prepared by the pulverizing method is susceptible to the ambient temperature and humidity. More specifically, at low humidities, the charge quantity of the toner is excessively increased, while at high humidities, the charge quantity of the toner becomes insufficient for use in practice.
In the case of the toner prepared by the polymerization method, a large quantity of a charge controlling agent and other agents can be contained in the polymerizable monomers, so that the environmental stability of the charging characteristics of the toner can be controlled to a certain extent. However, it is difficult to sufficiently disperse the charge controlling agent on the surface of the toner particles, so that there is also the problem that the charge quantity of the toner becomes excessive at low humidities and insufficient at high humidities.
These conventional toners prepared by the pulverizing method and polymerization method cannot always be provided with the desired charge quantity in a stable manner under various environmental conditions.
In addition, it has been proposed to decrease the molecular weight of a binder resin for use in a color toner in order to attain the image fixing at low temperature and to decrease the melt viscosity of the toner, as disclosed in Japanese Laid-Open Patent Applications 53-96839, 60-244956, 60-252363, 61-117564, 61-123854, 61-200549, 62-9356, 1-201672 and 3-185458. However, when the toners proposed in these applications are used, the problems of insufficient gloss and curling of the obtained toner images remain unsolved.
Most toners currently employed, especially color toners, are still not satisfactory in the above-mentioned respects.